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You’ve heard those worm horror stories, right? Stories of painful stomach cramps or diarrhea or nausea that eventually turns out to be caused by some worms that have taken up residence in someone’s intestines. It’s so terrifying and wild to think of something so much smaller than us causing so much havoc. But, what if worms had to worry about their own guts being taken over by a parasite?

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You’ve heard some worm horror stories, right?  We were looking some up just for this episode and   came across a recent headline from ArsTechnica that read,  “Army of worm larvae hatch from man’s  bum, visibly slither under his skin,”. And if that’s not enough to terrify you, and make you feel very uncomfortable there’s always the stories of painful stomach  cramps or diarrhea or nausea that eventually   turns out to be caused by some worms that have  taken up residence in someone’s intestines.  It is terrifying and wild to think of something  so much smaller than us causing so much havoc.   So, as we watch the cilia lining a worm’s gut  beat its own soothing pattern, wouldn’t it feel like, almost like, a little bit of justice if this sight  wasn’t so peaceful?

If worms had to worry about   their own guts being taken over by a parasite? If you’ve found yourself in this position,   seeking schadenfreude over a worm, well we  have some good news for you. The worm you see   in the middle of this tank is currently hosting  this strange fellow, called a Radiophyra.  James, our master of microscopes, had been on  the hunt for the Radiophyra after seeing this:   two radiophyras linked together in a chain  as one divided to make more copies of itself.   It had come from the inside of one of  the worms he’d been watching, when he’d   accidentally squeezed a worm a bit too hard under  the coverslip and caused the ciliate to pop out.  Radiophyra belong to a general  group of ciliates called Astomes,   or astomatid ciliates.

We’ve talked about ciliates  a lot on our channel, which means that if you’ve   been watching us for a while, you may have  picked up on the fact that from time to time,   we have said that most ciliates have an oral groove,  that opening lined with cilia that sweep bacteria   and algae and other tiny bits of food into the  organism. We’ve seen that oral groove at work   in ciliates like stentors and paramecium,  functioning as the ciliate equivalent of a mouth.  But as we have always said most ciliates, you will have inferred, that this does not mean all ciliates. And if you are looking for an exception to the rule, astomes are that exception.

Astomatid  ciliates are diverse, but they are unified by   one shared feature, or rather, they are unified  by their lack of one shared feature, a mouth.  And they don’t need a mouth because they  have something even better. They have worms.  Astomatid ciliates do parasitize animals  other than worms. Some live inside mollusks,   others inside leeches or even in amphibians.

But  they are most commonly associated with the guts of   annelid worms. So when James found his Radiophyra,  he decided to see if he could find more of them   in the other worms that were in his samples. And  that meant that our master of microscopes had to   become a master of worm surgery, dissecting them  so he could draw out the ciliates living within.  In–side this particular aquatic worm were  these astomatid ciliates.

From a distance,   they also look like worms. But as you get closer…  And closer, their shape becomes more definite  except for the massive amounts of fluff around   them, a dense cloud of cilia beating away. Unfortunately, there isn’t a lot of research   on this ciliate.

In fact, there isn’t a lot  of research on astomatid ciliates in general.   They just aren’t destructive enough or common  enough to have become either a necessary or   convenient research subject. In fact, it’s  not even clear whether or not we should call   them parasites. Modern day papers will sometimes  refer to them endosymbionts instead, because we   don't know a lot about whether astomatid ciliates  are doing much to their worm hosts, bad or good.  But the worm gut does plenty for the  astomatid ciliates.

At one point in time,   the ancestors of these ciliates did have  mouths. But as they found their way into worms,   and specifically their guts, those oral  grooves became less and less necessary.  Instead, the ciliates could rely on a form of  feeding called osmotrophy, where they simply   absorb nutrients from their surroundings through  osmosis. Instead of taking in larger bits of   food through their mouths and breaking it down  themselves, astomatid ciliates could just take   advantage of the worm’s digestive system to do  all that breaking down for them.

As the worm’s  digestive enzymes break down complex molecules into simpler forms  that can travel through their own intestinal walls,  some of those nutrients would just go  feed the astomatid ciliate instead.  These ciliates will actually sometimes be picky about  making sure they’re in a particular spot   within their host’s intestinal tract.  And once they’ve found the right spot,   the ciliates hold themselves in place with  organelles that range in shape, some use hooks,   other spines, or maybe even suckers. The flat  shape of the ciliates helps them stay pressed to   the epithelium of the intestines. So while these  ciliates may not need a mouth anymore, they have   found other traits necessary to their survival.

Astomatid ciliates are found in hosts from all   sorts of environments. Some live in soil.  Some live in ponds. Some even live in ocean   waters.

And scientists are using the general tools  available now to try and piece together how host   and endosymbiont have shaped each other.  We can see some of that intertwined story   in the ciliate’s mouthless-ness, but the  specifics of that evolutionary change   are laid out in their DNA, as are the other  more hidden parts of that shared history.  Now we cannot know whether worms have any  feelings about their guts being home   to another organism. They likely don’t have much  choice in the matter, and they also likely aren’t   prone to emotions like resentment the way we  might resent a worm parasitizing our bodies.  But if they were to have any feelings on  the matter, we want to offer this one last   detail about astomatid ciliates: many of  them have their own endosymbionts as well,   bacteria that live inside them with perhaps their  own history tangled up with their ciliate host’s   story as well. Now we couldn’t find those  bacteria in our parasitic friends,   but we like the idea that somewhere out there are  these nesting dolls of endosymbiosis: an organism   that is an organism but an organism that is also a  home buried within other homes.   Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.  And thank you to Brilliant  for sponsoring this episode.  Brilliant features hands-on,  interactive courses in science,   engineering, computer science and math.

And Brilliant has recently upped the interactivity   in their courses. In their Scientific Thinking  course, you’ll dive into the world of scientific   principles by exploring the laws of physics and  principles of engineering. Along the way you’ll   gain the understanding and insight needed to  start looking at the world in a different way.  The courses are designed for people  of all levels, so you can jump in   at any point and work your way to mastery.

And  Brilliant courses are also available offline   using their iOS and Android app. So if you’re  traveling or have a spotty internet connection,   you’ll be able to keep learning. If you’re interested in learning more,   you can get 20% off an annual premium  subscription at

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